Recent Investigations Towards Achieving a One Centimeter Geoid

Recent Investigations Towards Achieving a One Centimeter Geoid

Daniel R. Roman & Dru A. Smith

U.S. National Geodetic Survey

GGG 2000, Session 9

The Challenge of the cm-geoid

August 4, 2000

12:10-12:30

OUTLINE

• G99SSS/GEOID99 processing stream

• Refinements of hybrid methodology

• Development of a direct Helmert approach

• Other future developments

G99SSS/GEOID99 Processing Stream

• Use of Faye anomalies in G99SSS is same as approach taken for G96SSS (GEOID96)

• Higher resolution DEM in PNW increased calculated terrain correction (TC) signal

• TC’s calculated in much narrower bands

• More GPS on BM data improved regions previously under-represented (Midwest & Plains States) in GEOID96

Refinements of Hybrid Methodology

• G99SSS & GPS on BM residuals were modeled both by minimum curvature with a 300-km low-pass filter and by iterative LSC at 550 & 33 km correlation lengths

• Forthcoming models will be based on single-pass LSC using a 33 km correlation length and use of non-uniform sigma values based on GPS network accuracies

Development of a Direct Helmert Approach

• Terrain Effects broken into inner ( <4 degrees) and outer ( >4 degrees) zones

• Outer zone is being modeled using FFT’s

• Inner zone will be modeled in spherical coordinates in the space domain

• Merging will be with an equivalent taper applied in both space & frequency domains

• Downward continuation consistent with terrain effects is being explored

Other Future Developments

• Development of a North American geoid

• Impending satellite gravity missions

• Anticipated improvements to observations and DEM’s

Development of North American Geoid

• Joint (U.S., Canadian, Mexico, etc.) effort

• Anticipated scope is from the North Pole to the Equator including Greenland & Alaska

• Next NAG subcomm. meeting possibly in Mexico, Central America or Caribbean

• Largest obstacle remaining: Data gaps in Central America and Cuba

Impending Satellite Gravity Missions

• GRACE, CHAMP, & GOCE

• Expect cm-level commission errors for the resulting degree 120-250 harmonic models

• Combining these models with surface gravity observations should produce high-resolution geoid at the cm-level

• May eliminate the need for hybrid geoids and permit analysis of leveling networks

Anticipated Improvements to Observations and DEM’s

• National Adjustment of CORS Network in 2004 will create more cohesive GPS-derived ellipsoidal heights at bench marks

• Increased DEM resolution (1 arc-second for all of CONUS) will improve determination of 3D masses beneath gravity observations

Conclusion

• Ongoing research will improve current approach (near term) and develop avenues for more rigorous solutions (far term)

• Expected quality and quantity of future DEM and satellite gravity data sets should complement the theoretical improvements

• Development of a joint regional gravimetric geoid will enhance international studies